Heretofore, three different techniques have been proposed for increasing the framework Si/Al ratio of crystalline zeolites by the extraction of aluminum atoms and the at least partial substitution of silicon atoms into the sites previously occupied by these extracted aluminum atoms. Numerous chemical extraction procedures using mineral acids, chelating agents such as ethylene diaminetetracetic acid (EDTA), solutions of CrCl.sub.3, gaseous fluorine, phosgene and the like are also known, but the vacancies left behind by the extracted aluminum atoms are not reoccupied by silicon atoms. In the case of the CrCl.sub.3 treatment, however, the substitution of chromium atoms into the vacated sites is reported. In this regard see U.S. Pat. No. 3,937,791, Garwood et al.
Of the prior reported silicon insertion procedures, the earliest and the most thoroughly investigated involves the steaming of a hydrogen or ammonium exchanged form of the starting zeolite at temperatures usually in excess of 550.degree. C. using a steam environment containing at least 2 psia water vapor pressure.
For example, U.S. Pat. No. 3,591,488 discloses that the hydrogen or ammonium form of a zeolite may be treated with steam at a temperature ranging from about 800.degree. F. to about 1500.degree. F. (about 427.degree. C. to about 816.degree. C.), and thereafter cation-exchanged with cations including rare earth cations.
It appears to be generally accepted among those skilled in the art that aluminum atoms extracted from the crystal lattice and moved to the interstitial space are immediately replaced by a "nest" of four hydroxyl groups and that some portion of these hydroxyl nests are in turn replaced by silicon atoms. There is not general agreement as to the source of the replacing atoms, i.e., pre-existing framework silicon atoms or occluded silicon-containing impurities, or the degree to which "healing" of the lattice occurs. It has been shown, however, that steamed faujasite type zeolites develop a secondary mesopore system with pore radii in the range of 15 to 19 A, indicating that the silicon substitution mechanism does involve the migration of framework silicon atoms and possibly the elimination of entire sodalite cages. See in this regard, U. Lohse et al, Z. Anorg. Allg. Chem., 1980, 460, 179.
U.S. Pat. No. 3,493,519 discloses a process for calcining an ammonium-Y zeolite in the presence of rapidly-flowing steam followed by base exchange and treatment of the product with a chelating agent capable of combining with aluminum whereby aluminum is extracted from zeolite Y.
U.S. Pat. No. 4,036,739 discloses a hydrothermally stable and ammonia stable Y-type zeolite intended for use as a cracking catalyst. The zeolite is prepared by partial exchange of ammonium ions for sodium ions, steam calcination under controlled conditions of time, temperature and steam partial pressure, and a second ion-exchange of ammonium ions for sodium ions to reduce the final Na.sub.2 O content to below about 1 weight percent. Following the second ion-exchange, the zeolite is calcined for a time sufficient to effect substantial deammoniation but insufficient to reduce the unit cell dimension to below about 24.40 A. According to U.S. Pat. No. 3,781,199, the second calcination may be conducted after the zeolite is admixed with a refractory oxide.
A much later developed technique for silicon insertion is reported by Beyer et al in Catalysis by Zeolites, ed. B. Imelik et al (Elsevier, Amsterdam, 1980) p. 203. In this procedure, apparently operable only in the case of zeolites of the faujasite type of structure, the starting zeolite is contacted with silicon tetrachloride vapor at elevated temperatures, typically about 400.degree. C. to 500.degree. C. The reaction which occurs is ideally EQU Na.sub.x (AlO.sub.2).sub.2 (SiO.sub.2).sub.y +SiCl.sub.4 .fwdarw.Na.sub.x-1 (AlO.sub.2).sub.x-1 (SiO.sub.2).sub.y+1 +AlCl.sub.3 +NaCl
It is found that the high temperatures employed and the absence of appreciable water vapor during the course of the reaction results in a product zeolite having some silicon inserted into the crystal lattice but having a relatively low cation equivalence value, indicative of conventional decationization as reported in U.S. Pat. No. 3,130,006, Rabo et al.
The third type of silicon insertion technique heretofore proposed is reported in U.S. Pat. No. 4,503,023. In that patent specification Breck and Skeels describe a process for dealuminizing a zeolite by treatment of the zeolite with a fluorosilicate salt in an amount of at least 0.0075 moles per 100 grams of the zeolite (on an anhydrous basis), the fluorosilicate salt being provided in the form of an aqueous solution having a pH in the range of 3 to about 7. The aqueous solution of the fluorosilicate salt is brought into contact with the zeolite at a rate sufficiently slow to preserve at least 80 percent, preferably at least 90 percent, of the crystallinity of the starting zeolite and silicon atoms, as SiO.sub.4 tetrahedra, are inserted into the crystal lattice in substitution for aluminum atoms. The fluorosilicate extracts aluminum from the zeolite lattice framework and substitutes silicon therein, thus increasing the SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of the zeolite without introducing large numbers of defect sites in the framework. The products of this process are referred to as LZ-210.
As already discussed, most of the prior art processes for the dealumination of zeolites introduce so many defects into the lattice framework that the thermal stability of the zeolite is adversely affected. It is known that dealumination alone, which creates unhealed defect sites, does not increase the stability of the zeolite. It is the lattice framework rearrangement caused by the thermal or hydrothermal treatment that causes the stability of the zeolite to increase, as framework aluminum is removed and silicon fills some of the defect sites produced during dealumination. In the other two prior known silicon insertion techniques, it is the insertion of extraneous silicon atoms, i.e., atoms not derived from the zeolite itself, which prevents the formation of permanent defect sites in the crystal lattice and stabilizes the crystal structure. The method used to produce the LZ-210 zeolites appears to be the most effective means for the production of a silicon-enriched framework and one that maintains the ion-exchange capacity of the zeolite. Accordingly, other methods which produce a silicon-enriched framework and also maintain the ion-exchange capacity of the zeolite are desirable. The present invention provides such a method, in which a zeolite is treated with an aqueous solution of a bifluoride salt of which the reactive moiety is presumably the bifluoride ion.
The prior art contains numerous processes involving the treatment of zeolites with various types of halogen-containing compounds, including simple fluorides. These processes are generally intended to provide residual fluoride in the zeolite.
Such processes are described in:
U.S. Pat. No. 3,594,331, in which 2 to 22 grams of available fluoride are provided per 10,000 grams of zeolite to stabilize the zeolite;
U.S. Pat. No. 3,620,960 (treatment of the zeolite with molybdenum fluoride);
U.S. Pat. No. 3,630,965 (treatment of the zeolite with hydrofluoric acid);
U.S. Pat. No. 3,644,220 (treatment of the zeolite with volatile halides selected from the group consisting of aluminum, zirconium, titanium, tin, molybdenum, tungsten, chromium, vanadium, antimony, bismuth, iron, platinum group metals and rare earths);
U.S. Pat. No. 3,575,887 and U.S. Pat. No. 3,702,312 (treatment of the zeolite with fluorides and chlorides);
U.S. Pat. No. 3,699,056 (treatment of the zeolite with halogenated hydrocarbons);
U.S. Pat. No. 4,427,788 (ammoniacal aluminum fluoride solution for treatment of zeolite having silica/alumina ratio greater than 100); and
U.S. Pat. No. 4,427,790 (complex fluoroanion treatment of zeolite having a silica/alumina ratio greater than 100).
A variation of such fluoride treatment for zeolites is disclosed in U.S. Pat. No. 3,619,412, which describes treatment of a mixture of mordenite and amorphous silica-alumina with a solution of a fluorine compound such as ammonium fluoride or hydrofluoric acid. The hydrofluoric acid treatment is said to provide stability to mordenite-containing catalysts.
Other processes involving treatments of zeolites having silica/alumina ratios greater than 100 are disclosed in U.S. Pat. No. Nos. 4,427,786; 4,427,787; 4,427,789 and 4,427,791. U.S. Pat. No. 4,427,786 discloses the treatment of supported zeolites with boron trifluoride, hydrolysis of the boron trifluoride, an ammonium salt exchange and calcination. A comparison of Examples 2 and 9 of U.S. P. 4,427,786 shows that the catalytic cracking activity of zeolites having a silica/alumina ratio of less than 70 showed a decrease as a result of this process. U.S. Pat. No. 4,427,787 discloses the treatment of an alumina-supported zeolite with a dilute aqueous solution of hydrogen fluoride; this treatment is claimed to preferentially increase the catalytic cracking activity of zeolites having silica/alumina ratios over 100. U.S. Pat. No. 4,427,789 discloses the treatment of an alumina-supported zeolite with an aqueous solution of an alkali metal fluoride, impregnation with a warm solution of an ammonium salt and calcination. U.S. Pat. No. 4,427,791 discloses a process for the treatment of an inorganic oxide material with ammonium fluoride or boron fluoride, ammonium ion exchange, and calcination. This treatment is claimed to enhance the activity of the inorganic oxide material as a result of the ammonium ion-exchange step.
U.S. Pat. No. 4,324,698 discloses a process for preparing a fluorided zeolite type cracking catalyst consisting essentially of at least 90 percent by weight of a composite of a Y-zeolite in a silica-alumina matrix and containing about 0.1 to about 5 percent by weight of fluorine. The catalyst is prepared by contacting the composite with an amount of aqueous fluorine compound just sufficient for incipient wetting of the composite. The composite is then dried and calcined. The catalyst necessarily contains high concentrations of fluoride.